The design of conventional cameras is based on bionic study of the vertebrate eye (single-aperture optics), which can provide a faithful rendering of the visual world with a high resolution. However, it is difficult to obtain a wide field of view and extract motion-related information by such vision systems.
In contrast to the vertebrate eye, the insect compound eye consists of a curved microlens array which comprises a plurality of microlenses provided on a curved surface. This is very efficient for local and global motion analysis over a large field-of-view (FOV) although it offers a comparatively lower resolution.
The advantages of a microlens array (or compound eye) which comprises a plurality of microlenses provided on a curved surface include: a very large FOV, high motion sensitivity, accurate and fast navigation in three-dimensional (3D) dynamic environments, small volume, light weight, nearly infinite depth-of-field (DOF) AND SO ON. The microlens array (MLA) is a useful optical element with wide applications in liquid crystal display, wavefront sensor, image recorder, optical signal processing, and interconnection, etc.
Due to the above-mentioned advantages, an artificial vision system based on a compound MLA will be a radically different alternative to conventional cameras and will provide more efficient visual ability for large FOV imaging and motion analysis.
The fabrication of MLAs in a low cost, highly efficient and highly reliable way is of great interest to many engineering applications. A number of methods have been reported so far. Apart from conventional injection moulding, fluidic lens, slow tool servo method and roller imprinting are some among others. All of these methods are more applicable for larger sized lenses (i.e., where lens diameters are in the millimeter and above scale). It is difficult to make micro sized lenses (i.e., with a diameter less than 100 μm) due to reasons, such as high flow resistance during micro-scale injection moulding (i.e. filling in micro channel/mould), difficulties in master mould fabrication, soft lithography, ultraprecision machining and position control for 3D and micro-scale device fabrication, and expensive and complex procedure for microlens fabrication by conventional semiconductor processing methods.
Some special techniques have been developed for fabricating small sized lens arrays, such as ink-jet printing, soft lithography, electrohydrodynamic instability method, and some novel liquid/solvent approaches, etc. However, all these methods require the fabrication of a pre-patterned mould/mask by semiconductor processing techniques (e.g. lithography and etching), which is expensive and complicated.
The fabrication of compound microlens arrays which comprise a plurality of microlenses provided on a curved surface in a simple and cost-effective way is therefore still in great demand.